The proposed work in the avian system is designed to investigate specific aspects of T and B cell differentiation in normal and abnormal conditions, to further define the existence of separate lineages of precursor B (PB) cell populations in embryonic as well as adult hematopoietic tissues by characterizing functional parameters of these cells, and to elucidate mechanisms of specific modes of B and T cell activation through the combined use of cell separation techniques, chromosome markers and in vitro and in vivo culture methods.